A Mule and its Anis


From southern Mexico to northern South America, one of the first birds a birder new to this part of the world will check off is almost invariably the Groove-billed Ani (Crotophaga sulcirostris).  Common in the kinds of disturbed environments nearly every visitor passes through and moving relatively slowly but noisily about in groups close to the ground, they’re hard to overlook.

Groove-billed anis have also been the focus of scientific research.  The famous Costa Rican naturalist and ornithologist Alexander Skutch first explored their natural history while groundbreaking behavioral biologist Sandra Lee Vehrencamp detailed their fascinating communal breeding system.  Others too have studied these birds.

The relative ease with which we can observe the birds and the fact that some very good researchers have done just that would suggest that we would have a solid handle on the basic natural history of these birds.  But there is one aspect of these birds’ behavior that’s been somewhat of a mystery.  Interestingly, it’s probably a mystery that countless campesinos could have answered for us had they wherewithal and any idea that anyone cared!

We have long known that Groove-billed Anis associate with cattle, horses, and mules. What hasn’t been clear are the forms this association takes.  We know that, like cattle egrets, Groove-billed anis occasionally follow large domestic mammals and catch the insects these animals scare out of hiding while wandering about.  Bent (1940) claimed, however, that Groove-billed Anis not only did this but also perched on cattle and even sometimes removed ticks.  Alexander Skutch, however, doubted that the birds did this. He suggested that people who thought they had seen this had confused anis with Giant Cowbirds, a similar-looking species well known for this kind of behavior.

In a 1953 study, Austin Rand examined Groove-billed Ani feeding rates away from and in association with cattle.  Despite several months of observation, Rand never saw anis perching on cattle.  This and the fact that the birds did not stay with the cattle led Rand to conclude that these birds were “much less attached” to their benefactors than are cattle egrets and cowbirds. Nevertheless, Rand mentioned that his son once observed an ani “pick a conspicuous tick” off an animal.  Whether it did this while perched or not, Rand doesn’t say.

Finally, in 1974 Eric Bolen added to this discussion in a one-page note to the Bulletin of the Texas Ornithological Society.  I don’t have access to this note but Bonnie Bowen at the Cornell Lab of Ornithology’s Bird’s of North America site suggests that Bowen observed Groove-billed Anis feeding on arthropods attached to cows with these arthropods likely being ticks.

Recently, I watched a group of Groove-billed Anis catching flies that were on or flying about a tethered mule (I have also seen them doing the same on a nearby tethered horse), particularly its neck and side. Most of the flies — many of which visited me too — seemed to be house flies.

The group of anis attending the mule included one or more birds off of the mule and one bird perched on its crest.

The birds that weren’t on the mule perched on various nearby objects or hopped along on the ground after the mule. This last set of birds occasionally captured insects scared up by the mule but more often focused on the flies that were around the mule’s legs.  But what all the birds really seemed to be doing was waiting for those times when the mule lowered its head to feed.  When it did this, the anis rushed in to catch flies that were on or flying around the mule’s face.

Only one bird at a time occupied the crest of the mule.  When a new bird flew onto the mule’s crest the previous occupant always left within a couple of seconds (I’ve seen two birds stay somewhat longer on the larger crest of horse until one of the birds chased the other off).

While on the crest, birds actively hunted flies on both sides of the mule.

Perched birds often jumped off the mule when it lowered its head to the ground. While this could be because birds catch flies more easily from the ground, it more likely reflects the birds’ ability to hold on when the mule lowers its head. Birds on the horse that I observed did not jump off when the horse lowered its head even for extended periods of time.  The horse, however, had a natural flowing mane while the mule sported a perhaps somewhat more difficult to hold on to stand-up trim.

Rand argues that the benefits to anis of associating with cattle as opposed to foraging away from them is greatest during the dry season when insect populations are low. Certainly it is deep into the dry season here in Jalisco.

It’s interesting, though, that Rand never saw birds perching on cattle.  Possibly there were too few flies around the face and neck of these animals to make this strategy profitable. Fly density on domestic animals varies with such things as proximity to stables and cattle density.  The fact that the mule that I looked at was tethered also probably increased the density of flies compared to that found in a free-roaming animal.

While the mule had ticks, I never observed birds removing them.  This could be because the birds weren’t interested in them.  It could also be because the ability of birds to forage for ticks is limited by their ability to reach them.  To maintain their position on the mule, perched birds seemed very dependent on the hair on the crest of the mule. Possibly the only ticks anis can eat on the mule I observed are those near the animal’s crest and those around its legs.

References and Additional Sources

Bent, A. C. 1940. Life Histories of North American Cuckoo, Goatsuckers, Hummingbirds and their Allies. U.S. Natl. Mus., Bull., 176.

Bolen, E. G. 1974. A note on the foraging behavior of Groove-billed Anis. Bull. Texas Ornithol. Soc. no. 7:8.

Bowen, B. S., R. R. Koford and S. L. Vehrencamp. 1989. Dispersal in the communally breeding Groove-billed Ani (Crotophaga sulcirostris). Condor no. 91:52-64.

Bowen, B. S., R. R. Koford and S. L. Vehrencamp. 1991. Seasonal pattern of reverse mounting in the Groove-billed Ani (Crotophaga sulcirostris). Condor no. 93:159-163.

Bowen, Bonnie S.. (2002). Groove-billed Ani (Crotophaga sulcirostris), The Birds of North America (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/grbani

Rand, A. L. 1953. Factors affecting feeding rates of anis. Auk no. 70:26-30.

Skutch, A. F. 1959b. Life history of the Groove-billed Ani. Auk no. 76:281-317.

Vehrencamp, S. L. 1977. Relative fecundity and parental effort in communally nesting anis, Crotophaga sulcirostris. Science no. 197:403-405.

Vehrencamp, S. L. 1978. The adaptive significance of communal nesting in Groove-billed Anis (Crotophaga sulcirostris). Behav. Ecol. Sociobiol. no. 4:1-33.

Vehrencamp, S. L., B. S. Bowen and R. R. Koford. 1986. Breeding roles and pairing patterns within communal groups of Groove-billed Anis. Anim. Behav. no. 34:347-366.

Vehrencamp, S. L., R. R. Koford and B. S. Bowen. 1988. “The effect of breeding-unit size on fitness components in Groove-billed Anis.” In Reproductive success., edited by T. H. Clutton-Brock, 291-304. Chicago, IL: Univ. of Chicago Press.

Instructor of Tropical Rainforest and Canopy Ecology for the Institute for Tropical Ecology and Conservation (ITEC).

Leave a Reply

Your email address will not be published. Required fields are marked *